The present invention provides a method of performing an instantaneous moisture concentration measurement of a gas having a rapidly changing moisture concentration. Additionally, the present invention relates to a method of determining the drydown characteristics of an environment by performing a plurality of instantaneous moisture concentration measurements of a purge gas used in drying the environment. More particularly, the present invention relates to such methods in which the instantaneous moisture concentration of the gas is determined through comparison with a standard gas having a known moisture concentration.
It is required that in some processes, semiconductors be manufactured in environments containing essentially no moisture. The moisture level in such manufacturing environments is however, not always stable. For instance, process chambers of rapid thermal processers are exposed to room ambient during the process loading cycle. After the loading cycle, the moisture concentration inside of the process chamber is reduced by introduction of purge gas into the process chamber. The changing moisture concentration of such an environment, termed in the art as the "drydown" characteristic of the chamber, is visualized by plotting a curve of purge exhaust gas moisture concentration versus time.
A typical method of defining such a curve is to connect a probe of a moisture analyzer to an exhaust outlet of the process chamber and to simply plot a curve of the probe output verses time after door closure. Using this method, the probe is also exposed to the rapidly decreasing moisture concentration of the purge gas and thus, its drydown characteristics will effect the accuracy of the curve. Put quite simply, the moisture concentration of the purge gas emanating from the chamber exhaust outlet can change far more rapidly than the probe is able to respond to such change.
As will be discussed, the present invention does not involve direct measurement of the moisture concentration of a gas. Rather, it allows for instantaneous concentration measurements to be made that are not dependent upon the response characteristics of the moisture analyzer used in making the measurements. A series of such measurements can then be used to generate a drydown curve of for instance, the process chamber of a rapid thermal processer.